Diode-type CMOS imagers for terahertz radiation [laserfocusworld.com] have been built. Those convert terahertz radiation into DC, which can then be amplified by standard techniques. But diodes don't have gain. That's why the original article emphasizes that this new device has gain.

There are terahertz lasers, waveguides [sandia.gov], antennas, and other components that work up there. The situation is much like radar during WWII; there were a few components that could do specific things at radar frequencies (then 60MHz to 1.2GHz), but general electronics wasn't there yet. Most of the electronics in radars of that period ran at far lower speeds. They still worked.

Incorrect. Gain is unitless (Vout/Vin), and decreases pretty much proportionally to the inverse of the frequency on amplifiers, so chip makes use something called the Gain Bandwidth Product (GBP) instead of the 'pure' gain, because it is a much more useful number for specifying actual transistor/amplifier performance in real live working conditions. And the unit of the GBP, is, you guessed it! Hz. Thus, a transistor with a GBP of 0.85THz will have a gain of 1 at 0.85THz, a gain of 2 at 0.425THz etc. When I see a gain with units in Hz I subconsciously think 'GBP' and don't even miss a beat...See http://en.wikipedia.org/wiki/Gain%E2%80%93bandwidth_product [wikipedia.org] if my explanation doesn't make sense.